<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        *{
            margin: 0px;
            padding: 0px;
        }
        div{
            width: 800px;
            height: 300px;
            background-color: #ccc;
        }
        p{
            width: 200px;
            height: 200px;
            background-color: red;
            margin-right: 50px;
            float: left;
        }
    </style>
    <link rel="stylesheet" media="screen and (min-width: 700px) and (max-width: 800px)" href="./big.css">
    <link rel="stylesheet" media="screen and (min-width: 400px) and (max-width: 700px)" href="./middle.css">
    <link rel="stylesheet" media="screen and (max-width: 400px)" href="./small.css">
</head>
<body>
    <div>
        <p></p>
        <p></p>
        <p></p>
    </div>
</body>
</html>